The present invention relates to a reflector-provided dipole antenna suitable for an antenna of a base station for mobile communication, or the like, for which broad-band characteristics and simultaneous transmission and reception are required.
For base stations for mobile communication, particularly for a cellular telephone system, a communication system with small radio zones is used. In such a system, the same frequency is reused in different radio zones in order to efficiently use the precious frequency resources and to cope with the extreme increase of subscribers.
In the cellular telephone system, the space diversity technique is generally used to improve the communication quality. As a result, however, the number of antennas installed at each base station increases. For decreasing the number of antennas, the same antenna is used in common for the transmission and reception in the uplink and downlink with different frequencies.
For this type of transmitting-receiving antenna, it is required to have as wide a frequency band as 810 to 960 MHz for the digital cellular telephone system, an example in Japan. Moreover, it is required that horizontal directivities for transmission and reception are almost identical in the uplink and downlink so that the service quality in both links is kept equal.
FIG. 20 is a perspective view showing a reflector-provided dipole antenna having been used so far as an antenna meeting this requirement, in which symbol 1 denotes a reflector and 2 denotes a dielectric substrate.
Symbol 3 denotes a conductor for forming a dipole antenna element and 4 denotes an earth conductor which is perpendicularly attached to the central portion of the said conductor 3. Both 3 and 4 are provided on the back surface of the dielectric substrate 2.
FIG. 21(a) shows an essential portion of the back side of the dipole antenna element and feed circuit. A notch 20 is formed at one side of the central portion of the conductor 3, dividing the conductor 3 into two, and forms a dipole antenna element. From the vertex of the notch 20, a slot 21 is formed on and along the earth conductor 4 for forming a feed circuit for the dipole antenna element. The intersection 22 of notch 20 and slot 21 is the feed point of the dipole antenna element.
FIG. 21(b) shows the face side of substrate 2. Symbol 5 denotes a folded conductor for forming a feed circuit and 16 denotes a parasitic element. The folded conductor 5 forms a micro-strip line balance-unbalance conversion circuit (BALUN) associated with the branch conductor formed by the divided portion of the earth conductor 4 provided on the back of the dielectric substrate 2.
The parasitic element 16 includes a linear conductor having a length a little shorter than .lambda./2, .lambda. being the designated radiation wavelength, and is set a little separated from and in parallel with the conductor 3, which forms a dipole antenna element as shown in FIG. 20. Those elements such as 5 and 16 formed on the face side of substrate 2 can be formed on the back side, provided that elements such as 3 and 4 on the back side are formed on the face side.
In FIG. 20, a coaxial connector (not shown in the figure) is provided on the back of the reflector 1. The inner conductor of the coaxial connector is made to pass through a hole provided on the reflector 1 and connected to the rear end of the folded conductor 5 so that it is not electrically connected with the reflector 11 and the outer conductor of the coaxial connector is connected to the rear end of the earth conductor 4 through the hole on the reflector 1.
In the case of this antenna, the resonance characteristics of the dipole antenna element including the conductor 3 is electromagnetically coupled with the resonance characteristics of the parasitic element 16 and broad-band characteristics are obtained based on the double-tuning principle.
FIG. 22 shows the frequency response of beam width (half power beam width) of the conventional antenna shown in FIGS. 20, 21(a) and 21(b) in the magnetic field plane (in the horizontal plane when the radiated wave is of vertical Polarization) for the case where the distance between the feed point and the reflection surface of the reflector 1 is approximately 0.3.lambda., the length of the reflector 1 in the direction of electric field is approximately 1.lambda., and the length of the reflector 1 in the direction of magnetic field on the reflector surface is approximately 0.6.lambda..
In this figure, the x-axis shows the frequency (MHz) of the radiated wave and the y-axis shows the beam width (degree) in the magnetic field plane. From FIG. 22, it is found that there is a defect in that the beam width changes greatly in accordance with the change of frequency of the radiated wave.
In the case of the conventional antenna shown in FIGS. 20, 21(a) and 21(b), the parasitic element 16 is made so that the length of the element 16 is a little shorter than that of the dipole antenna element. Therefore, the conventional antenna resonates at a frequency higher than that of the dipole antenna element. Thus, the beam width decreases when the frequency of radiation wave is low because the parasitic element 16 serves as a director. When the frequency of radiation wave is high, large current flows through the dipole antenna element serving as the radiation center and the large current flowing through the dipole antenna element moves to the parasitic element and the beam width increases.
That is, the parasitic element of a conventional antenna is effective to widen the bandwidth of return loss but it is improper as a shared antenna for uplink and downlink having different frequencies like an antenna of a base station for mobile communication because the beam width greatly changes against the frequency change of radiation wave.
Moreover, in the case of a conventional antenna, the parasitic element must be placed a little separated from the dipole antenna element for keeping the wide band characteristic, and the height of substrate 2 from the reflector 1 becomes large. Therefore, the outside diameter of a cylindrical radome for covering the antenna must be increased and thereby, difficulty arises in selecting the place for antenna installation due to the increased weight, size, and wind load of the radome.